Change these parameters all to any username you like to access, for me I changed all the parameters from nagiosadmin to admin or just add admin with a comma after nagiosadmin:
1. authorized_for_system_information
2. authorized_for_configuration_information
3. authorized_for_system_commands
4. authorized_for_all_service_commands
5. authorized_for_all_host_commands

This step is not necessary however i felt nagiosadmin is a bit long for username.

500 internal server error
If you encounter the 500 internal server error it could be because the htpasswd.users is not located, check nagios.cfg to see if the htpasswd path is the same you have provided using the htpasswd -c command.